The invention herein described provides modification of a conventional GPS receiver to provide accurate and timely location information on objects not colocated with the GPS receiver. For general background purposes, there are two sources which are recommended: 1) "Understanding GPS Principles and Applications" by Elliot Kaplan, Artech House, 1996, and 2) "Bistatic Radar", by Nicholas Willis, Artech House, 1991.
There are myriad patents relating to use of GPS to find location, attitude, velocity, etc., of platforms upon which GPS sensors are mounted, for example patents: U.S. Pat. Nos. [5,043,736] [4,894,662] [4,870,422] [4,797,677]. All of these systems utilize a GPS direct path signal (a signal which proceeds directly from each GPS satellite to the GPS receiver) in order to determine position of the GPS receiver via multi-lateration.
The GPS signals include at least one carrier signal, which may be at a frequency of around 1 gHz or so, with a known pseudorandom noise sequence (PRN) and a message signal impressed on the carrier signal. The PRN sequence from several satellites (in the GPS system at least 4) is used to determine exact time of transmission of signals from any satellite, while the message signal carries ephemeris (satellite position) and other information. The PRN sequence, in the C/A (civilian) version of GPS is a free- running PRN sequence that repeats every 1 millisecond or so, greatly facilitating a receiver's ability to lock onto the signal. Additionally, in the C/A version, the PRN code has a baud rate of about 1 mHz or so. The message code impressed on the PRN sequence is slow, with a baud rate of about 50 Hz or so. In contrast, in the military version of GPS, the PRN code is encoded, and if left in a free-running state, would take almost a year to repeat. As a practical matter, the code is reset every week or so. Also, the P(Y) military baud rate is about 10 mHz, an order of magnitude higher than the C/A codes.
In standard GPS and GLONASS concepts, receiver position is refined by successive determinations of range from multiple GPS or GLONASS satellites. For example, determination of range of a receiver from a single GPS satellite (S1), will narrow receiver position uncertainty down to the surface of a sphere of radius R1 (with some thickness which relates to the pseudorange accuracy), the sphere centered on the satellite location (known via ephemeris data). Determination of range (R2) of the same receiver to another satellite (S2) will result in further refinement of receiver position, within limits of the pseudorange accuracy, to an intersection of the two spheres of radius R1 and R2 centered on satellites S1 and S2, respectively. By extending this geometric location approach to include additional satellites, conventional GPS receivers provide receiver location via pseudorange when ranges to four or more satellites are known (three satellites are required for location, but an additional satellite signal is required for time correction). The present invention uses a GPS, GLONASS or possibly other RF source multipath signal scattered from objects of interest to determine [x,y,z] coordinates of these objects with respect to position of a receiver modified in accordance with the instant invention Using relatively long integration times (in excess of 20 ms) of data bit changes of the PRN code, which are implemented by phase shifts, the use of data wipeoff is required, which will be explained further hereinafter, to correct for phase shifts of the message signal in the PRN sequence, but results in a viable Signal to Noise Ratio (SNR) for targets at ranges of interest. This search technique increases SNR in two ways: 1) through signal processing gain induced by coherent chip integration of correlators involved with signal processing gain, and 2) through reduction of noise bandwidth due to pre-detection bandwidth being approximately equal to the reciprocal of integration time. This search strategy can be processing intensive if carried out over many range/Doppler bins, but is easily amenable to parallel processing. Once range/Doppler search has been achieved over the desired velocity/range interval, range/Doppler bins may be compared to a threshold determined by statistical detection to achieve a given probability of detection and false alarm rate. Range/Doppler bins achieving detection can then be placed in search loops (PRN sequence and carrier) analogous to those currently used in standard GPS receivers in order to achieve and maintain lock on direct path GPS signals. Since integration times longer than those employed in a standard GPS receiver will be necessary in order to detect and track indirect path targets, track update rate will be lower than desirable. Track update rate may be artificially increased at the expense of further parallel processing and memory, wherein a sliding integration window is used to maintain track with a constant and relatively long integration time while maintaining a short update rate.
The following two patents, which may be relevant to Applicant's disclosure, discuss uses of scattered GPS signals: U.S. Pat. No. 5,546,087 by Neira, M. M., and U.S. Pat. No. 5,187,485 by Tsui et al. The Neira patent utilizes the multipath signal scattered from the ground coupled with a theoretical model of the terrestrial sphere in order to provide precise altimetry measurements. The method used in the Neira patent is intrinsically based upon the assumption that the target of interest resides on the Earth's surface, a constraint not needed in the instant invention. The Tsui patent seeks to passively determine range to targets via use of scattered GPS signals. While solutions are postulated for target three dimensional coordinates in the Tsui patent, there is an inherent constraint that angle of arrival (AOA) of a multipath signal from a target to a receiving station must be known. In the present invention, the AOA information may be used, but is not required for a three dimensional solution to fix target position.